1. Field of the Invention
The present invention relates generally to riser tensioner systems for use on offshore platforms and, more particularly, to a riser tensioner system that provides a variable spring rate to maintain a substantially constant upward force on a supported riser.
2. Description of Related Art
Increased oil consumption and rising oil prices have lead to exploration drilling and production in geographic locations that were previously considered to be economically unfeasible. As is to be expected, drilling and production under these difficult conditions leads to problems that are not present under more ideal conditions. For example, an increasing number of facilities are located in offshore locations in order to tap more oil and gas reservoirs. These exploratory wells are generally drilled and then brought into production from floating platforms that produce a set of problems peculiar to the offshore drilling and production environment.
Offshore drilling and production operations require the use of pipe strings that extend from equipment on the sea floor to the floating platform. These vertical pipe strings, typically called risers, convey materials and fluids from the sea floor to the platform, and vise versa, as the particular application requires. The lower end of the riser is connected to the well head assembly adjacent the ocean floor, and the upper end usually extends through a centrally located opening in the hull of the floating platform.
As drilling and production operations progress into deeper waters, the length of the riser increases. Consequently, its unsupported weight also increases. Structural failure of the riser may result if compressive stresses in the elements of the riser exceed the metallurgical limitations of the riser material. Therefore, mechanisms have been devised in order to avoid this type of riser failure.
In an effort to minimize the compressive stresses and to eliminate, or at least postpone, structural failure, buoyancy or ballasting elements are attached to the submerged portion of the riser. These elements are usually comprised of syntactic foam elements, or of individual buoyancy or ballasting tanks, coupled to the outer surface of the riser sections. Unlike the foam elements, the tanks are capable of being selectively inflated with air or ballasted with water by using the floating vessel's air compression equipment. These buoyancy devices create upwardly directed forces in the riser and, thereby, partially compensate for the compressive stresses created by the weight of the riser. However, experience shows that these types of buoyancy devices do not adequately compensate for the compressive stresses or for other forces experienced by the riser.
To further compensate for the potentially destructive forces that attack the riser, the floating vessels incorporate other systems. Because the riser is fixedly secured at its lower end to the well head assembly, the floating vessel will move relative to the upper end of the riser due to wind, wave, and tide oscillations normally encountered in the offshore drilling environment. Typically, lateral excursions of the drilling vessel are prevented by a system of mooring lines and anchors or by a system of dynamic positioning thrusters that maintain the vessel in a position over the subsea well head assembly. Such positioning systems compensate for normal current and wind loading, and they prevent riser separation due to the vessel being pushed away from the well head location. However, these positioning systems do not prevent the floating vessels from oscillating upwardly and downwardly due to wave and tide oscillations. Therefore, the riser tensioning systems on the vessels are primarily adapted to maintain an upward tension on the riser throughout the range of longitudinal oscillations of the floating vessel. This type of mechanism applies an upward force to the upper end of the riser, usually by means of a cable, a sheave, or a pneumatic or hydraulic cylinder connected between the vessel and the upper end of the riser.
However, pneumatic and hydraulic tensioning systems are large, heavy, and require extensive support equipment. Such support equipment may include compressors, hydraulic fluid, reservoirs, piping, valves, pumps, accumulators, electric power, and control systems. The complexity of these systems necessitate extensive and frequent maintenance which, of course, results in high operating costs. For instance, many riser tensioners incorporate hydraulic actuators which stroke up and down in response to movements of the floating vessel. These active systems require a continuous supply of high pressure fluids for operation. Thus, a malfunction could eliminate the supply of this high pressure fluid, causing the system to fail. Of course, failure of the tensioner could cause at least a portion of the riser to collapse.
In an effort to overcome these problems, tensioner systems have been developed which rely on elastomeric springs. The elastomeric riser tensioner systems provide ease of installation, require minimal maintenance, and offer simple designs with few moving parts. These springs operate passively in that they do not require a constant input energy from an external source such as a generator. Moreover, the elastomeric systems do not burden the floating platform with an abundance of peripheral equipment that hydraulic systems need in order to function.
The elastomeric devices operate in the shear mode, whereby the rubber-like springs are deformed in the shear direction to store energy. The shear mode of operation has numerous shortcomings. For example, in the shear mode, rubber exhibits poor fatigue characteristics, which can result in sudden catastrophic failure. When numerous rubber springs are combined in series, the reliability of the system quickly deteriorates because only one flaw in the elastomeric load path can very quickly lead to catastrophic failure of the entire system.
Moreover, an ideal tensioner system provides a constant tensioning force to support the riser. While some of the complicated hydraulic systems alluded to above can be controlled to provide a substantially constant force, the simpler elastomeric devices which overcome many of the problems of the hydraulic systems do not support the riser using a constant force. Thus, changes in the force exerted on the riser in response to longitudinal excursions of the platform produce undesirable tensile stress fluctuations in the riser. These fluctuations can substantially shorten the useable life of the riser. In addition, most currently available elastomeric systems are quite complex and, thus, quite expensive.
The present invention is directed to overcoming, or at least minimizing, one or more of the problems set forth above.